The immune system is an especially attractive organ system in which to study the diminished proliferative capacity of cells in the elderly due to accessibility of cells and the clinical significance of immune function defects. The aged have a diminished production of interleukin 2 (IL-2), an essential growth factor for T cell proliferation. In the mouse, exogenous IL-2 was shown to largely correct many in vitro functional deficits. With human cells in vitro, however, this has not been effective; thus; other factors must also play important roles. To induce cell growth, IL-2 must interact with specific high affinity receptors (IL-2R) displayed on the surface of activated T cells. Such activated T cells also express low affinity IL-2 receptors and release a soluble from of IL-2R, neither of whose functions are known. Preliminary experiments demonstrate that the aged release a relative excess of soluble IL-2R for the level of mitogen-induced proliferation. The expression of membrane- bound IL-2R has not been well-characterized in the elderly, but in a limited number of donors, the aged appear to express a higher density of receptors per activated cell. This proposal is designed to investigate both membrane-bound and soluble IL-2R in healthy aging donors. membrane-bound IL- 2R will be studied utilizing 125I-labelled IL-2 to determine the number, affinity, and kinetics of expression of both high and low affinity receptor populations. Soluble IL-2R will be investigated by quantitating its release from stimulated T-cells. The origins of soluble IL-2R will also be examined in the old and young by intrinsic radiolabelling experiments and the use of proteolytic and lysosomal enzyme inhibitors. The functions of soluble IL-2R will be analyzed utilizing preparations of a recombinant DNA- produced IL-2R which has been modified to lack the transmembrane portion of the molecule, as well as preparations of soluble IL-2R crudely purified from culture supernatants. In vitro thymic hormones will be tested for effects on IL-2R expression in cells from the aged. Experimental methods for these projects include: a) cell separations using standard techniques, b) cell culture with a variety of lymphocyte activators, c) assay of membrane-bound IL-2R binding by radiolabelled IL-2, d) determination of soluble IL-2R by enzyme-linked immunosorbent assay, e) polyacrylamide gel electrophoresis, and f) gel permeation chromatography.